Use laptop A's LCD as secondary monitor for laptop B [duplicate]

Possible Duplicate:
Can I use my laptop as a second monitor for my desk computer?

I have a working laptop (laptop A), but am interested in using the laptop's LCD as a secondary monitor for my main laptop (laptop B)...is there a way to do this via software or cabling without modifying laptop A? OS's are Windows XP SP3 and Windows 7.

Update: Software must be free, and a bonus if I can switch between using Laptop A as a secondary monitor and its regular desktop.

There are options to make this happen: Here's a similar question with some ideas: Use an old laptop as a monitor?